Silicon carbide (SiC) is characterized in that a breakdown electric field is one digit greater than that of silicon (Si), and a band gap and thermal conductivity are about three times more than those of Si. Therefore, silicon carbide (SiC) is expected to be applied to, for example, power devices, high-frequency devices, and high-temperature operation devices.
A SiC epitaxial wafer is manufactured by growing a SiC epitaxial film serving as an active region of a SiC semiconductor device on a SiC single crystal substrate, in general, using a chemical vapor deposition (CVD) method, in which the SiC single crystal substrate is processed from a SiC bulk single crystal produced by, for example, a sublimation method.
A defect with a triangular shape (hereinafter referred to as a “triangular defect”) has been known as a defect in the SiC epitaxial film. The triangular defect is formed in a direction in which the apex of a triangle and an opposite side (base) are sequentially arranged in a line along a step-flow growth direction (NPL 3). That is, the opposite side (base) of the triangular defect is disposed in a direction perpendicular to the <11-20> direction. A plurality of origins of the triangular defect are considered. Examples of the origins include damage, such as a polishing flaw which remains on the surface of a substrate (wafer) (PTL 1), a 2-dimensional nucleus which is formed in a terrace during step-flow growth (PTL 2), a different polytype of crystal nucleus which is formed at the interface between the substrate and the epitaxial film in an oversaturated state at the early stage of growth (NPL 1), and a defect which has a minute broken piece of a SiC film, which will be described below, as a starting point. The triangular defect is grown as the SiC epitaxial film is grown. That is, the triangular defect is grown with step-flow growth such that its area increases while a shape which is substantially similar to a triangle and has the starting point as its apex is maintained (see the schematic diagram of FIG. 5). Therefore, in general, in as early stage of the growth of the SiC epitaxial film as the starting point of the defect occurs, the triangular defect grows large, and the depth of the starting point in the film can be estimated from the size of the triangular defect.
A reduction in the triangular defects is indispensable in order to improve yield in the mass production of the SiC epitaxial wafer. PTL 1 and PTL 2 disclose methods of reducing the triangular defects.
In addition, as a cause of deterioration in the quality of the SiC epitaxial film, there is a minute broken piece (hereinafter referred to as a “downfall”) of a SiC film which falls on a SiC wafer, in a SiC epitaxial film, or on the SiC epitaxial film. The downfall is a piece of a SiC film which peels off from the SiC film deposited on a ceiling that is provided on the upper side of the chamber so as to face the upper surface of a susceptor including a wafer mounting portion on which a SiC wafer (SiC substrate) is placed. The downfall can be the starting point of the triangular defect.
Here, when the SiC epitaxial film is grown, it is necessary to heat the SiC wafer, which is a substrate, at a high temperature and to maintain the temperature. As a method of heating the substrate and maintaining the temperature, a method has been mainly used which performs heating using heating means that is located below the susceptor and/or above the ceiling (see PTL 3, NPL 2, and NPL 3). When the ceiling is heated, it is generally heated by high-frequency induction heating using an induction coil. The ceiling which is made of carbon suitable for the high-frequency induction heating is generally used.
While the SiC epitaxial film is formed, SiC is deposited not only on the SiC wafer but also on the ceiling or other members which are placed in a chamber (SiC-CVD furnace). When the film is repeatedly formed, the amount of SiC deposited on, for example, the ceiling, increases. Therefore, the problem of the downfall becomes notable particularly in mass production.
A reduction in the downfall is indispensable in order to improve yield in the mass production of the SiC epitaxial wafer.
However, in the SiC epitaxial wafer, nitrogen also becomes a dopant. Therefore, when the SiC epitaxial wafer is manufactured, a chamber (SiC-CVD furnace) 200 is arranged in a glove box 100, as shown in FIG. 23. Then, the glove box is filled with inert gas, such as argon gas. In general, a film forming process which forms a SiC epitaxial film on a SiC wafer while circulating the inert gas in the glove box through a filter 300 provided in the glove box (turning on circulation) is performed to manufacture the SiC epitaxial wafer. An arrow connecting two filters 300 schematically indicates the circulation of the inert gas.
As the filter which is provided in the glove box, for example, a filter is used which has the minimum removal ratio (particle collection ratio) with respect to particles with a size of 0.3 μm and has a particle removal ratio of 99.97% or more with respect to particles with a size of 0.3 μm.
Whenever a cover 201 of the chamber (SiC-CVD furnace) is opened in order to place the SiC substrate or to withdraw the manufactured SiC epitaxial wafer, a deposit (particle), such as SiC, which is attached to the members which are placed in the chamber is scattered in the glove box and contaminates the inside of the glove box. The main purpose of the circulation of the inert gas in the glove box through the filter is to remove the deposit.
In FIG. 23, a dotted line above the cover which is represented by reference numeral 201 indicates a state in which the cover is opened and a vertical arrow indicates that the cover can be opened and closed.